A grind mill is a type of device for decreasing particle size of an input solid material, which has been widely used in a variety of industries such as the chemical and food industries. There are many types of grind mills including, but not limited to, a pin mill, a ball mill, a colloid mill, a conical mill, a disintegrator, a disk mill, an edge mill, and a hammer mill. The disc mill has two grind plates which rotate at different speeds. Solid particles pass through a gap between the two plates to decrease the particle size by grind plate action. If one grind plate is stationary and the other grind plate rotates, this is referred to as a single-disc mill. If both grind plates rotate but in opposite directions, it is referred to as a double-disc mill.
Typical applications for a single-disc mill are for wet milling processes such as in corn wet milling and the paper industry, manufacture of peanut butter, processing nut shells, ammonium nitrate, urea, producing chemical slurries and recycled paper slurries, and grinding chromium metal. Double-disc mills are typically used in the paper industry and as well other industry such as alloy powders, aluminum chips, bark, barley, borax, brake lining scrap, brass chips, sodium hydroxide, chemical salts, coconut shells, copper powder, cork, cottonseed hulls, pharmaceuticals, feathers, hops, leather, oilseed cakes, phosphates, rice, rosin, sawdust, and seeds.
In an exemplary application in the dry mill industry, corn is passed though a hammer mill to grind the corn to flour with wide particle size distribution, such as smaller than 45 micron to 3 mm size. Water is then added to liquefy the starch and convert to a sugar solution before sending to a fermenter to convert the sugar to alcohol. Some germ and grit particles with size larger than 200 micron need further grinding in the liquefaction step to further break up the solid particles to break the bond between starch/protein/oil/fiber in the germ and grit particles. Examples of such a a process can be found in patent application Ser. No. 13/428,263, entitled “Dry Grind Ethanol Production Process and System with Front End Milling Method”, which is hereby incorporated in its entirety by reference.
Two types of grind plates are the devil tooth design and the bar and groove design. FIG. 1 illustrates to down view of a grind surface of a conventional bar and groove grind plate design. FIG. 2 illustrates a top down view of a grind surface of a conventional devil tooth design. Both the bar and groove grind plate and the devil grind plate span 60 degrees, six such plate are positioned end to end to form a completed grind disc spanning 360 degrees. The grind discs are typically 36 inches or 52 inches in diameter. The exemplary bar and groove disc plate shown in FIG. 1 and the devil grind plate shown in FIG. 2 are for a 36 inch diameter grind disc. A 52 inch diameter grind disc can be formed using a single ring of six such grind plates, as described above, or alternatively using two separate rings. The first ring is formed using similar grind plates as those used to form the single ring, 36 inch diameter grind disc, and the second ring is formed around the first ring using twelve similar grind plates as the inner ring except each of the twelve outer ring grind plates spans 30 degrees. The inner edge of the outer ring grind plates are configured to mate to the outer edge of the inner ring grind plates. The bar and groove grind plates are normally used in the paper industry. The devil tooth grind plates are normally used in the corn mill industry and prove better than bar and groove grind plates in this application because devil tooth grind plates result in higher capacity and avoid producing too much fine fiber, as is the case with bar and groove grind plates.
The disc mill has two grind plates fitted together such that the grinding elements, for example the teeth of the devil tooth grind plate design, face each other. FIG. 3A illustrates a top down view of a grind plate A of a conventional devil tooth design used in the dry mill industry. FIG. 3B illustrates a side view of the grind plate A of FIG. 3A. The grind plate A is the first of two complementary grind plates used in a disc mill. FIG. 4A illustrates a top down view of a grind plate B of a conventional devil tooth design used in the dry mill industry. FIG. 4B illustrates a side view of the grind plate B of FIG. 4A. The grind plate B is the second plate of the disc mill and is the complement to grind plate A. The grinding surface of grind plate A shown in FIG. 3A faces the grinding surface of grind plate B shown in FIG. 4A such that row 1 of grind plate A is positioned between rows 1 and 2 of grind plate B, row 2 of grind plate A is positioned between rows 2 and 3 of grind plate B, and so on. The grind plates A and B are spaced by a gap to provide a solid path way through which the material to be ground can pass. The actual grind surface of the devil tooth grind plate design is considered the tooth side surface. The actual grind surface on a bar and groove grind plate design is considered the total bar surface. Comparing the actual grind surfaces of the two designs, the bar and groove grind plate design has an actual grind surface of around 350 square inches as compare with the devil tooth grind plate design that has an actual grind surface of around 570 square inches. The grind plate efficiency depends on the actual grind surface multiplied by the rotating tip speed of teeth. The grind capacity depends on a solid pass way open area with minimum gap between the teeth on opposite sides of the complementary grind plates.
FIG. 5A illustrates detailed design parameter values corresponding to the devil tooth grind plate design. The tooth variable L is the tooth length, the tooth variable W is the tooth width, the tooth variable H is the tooth height, the tooth variable A is the tooth front and back slope angle, and the tooth variable B is the tooth side slope angle. The grind plate variable N is the number of teeth on each row, the grind plate variable D is the distance between teeth on the same row, and the grind plate variable R is the number of rows on the grind plate. The conventional devil tooth grind plate is designed with teeth in adjacent rows substantially aligned or primarily aligned so that an open channel is formed, such as the straight channel view shown in FIG. 5B. FIG. 5C illustrates a cut out side view of the of the two complementary grind plates with the two grind plates touching. A plate gap P is defined as the distance between the tip of the teeth on one grind plate, such as grind plate A, and the surface of the other grind plate, such as grind plate B, opposite the tip of the teeth. A side gap G is the separation distance between the side surfaces of opposing teeth on the two grind plates.
The conventional devil tooth grind plate design has a number of disadvantages. First, the number of teeth on the inner rows, the rows closest to the center of the grind disc, decreases significantly compared to the number of teeth on the outer rows because there is a need for more open area for solid pass through. The fewer the number of teeth the lower the grinding capacity. Second, the tooth height H is too short, from 0.34 to 0.59 inches in conventional designs, and the tooth height H-to-tooth width W ratio is too low, from 0.4 to 0.7 for conventional designs. For high feed rate, the side gap G separating opposing tooth side surfaces are farther apart to give enough opening area for solid pass though. The greater the gap G, the less the overlapping grind surface from opposing tooth side surfaces. Third, the solid pass way open area on each row is not constant and results in braking action of the solid passing from the inner rows to the outer rows. This also results in additional power requirements. Fourth, the straight channel configuration of teeth from row to row does not block solid material from easily bypassing multiple rows without being ground. Fifth, the feed inlet design is not uniform and consistent, which leads to irregular input of solid material into the disc mill.